The novel apparatus and process of the present invention is particularly useful in the loading and/or unloading of silicon or other semiconductor wafers from wafer-holding cassettes to quartz boats before placing the wafers into a diffusion furnace.
The present invention provides an apparatus for transporting a wafer comprising a robot and a paddle or end-effector. The improvement comprises a sensor means disposed on the surface of the paddle, the sensor means producing electrical analog signals which represent the wafer position on the paddle, connected to a microprocessor operable to process digital signals, which have been converted from the analog signals.
A further feature of the present invention includes an apparatus for mapping the position of slots in the stacking rods of a quartz boat, thereby to establish reference points for the precise placement or positioning of wafers in the slots. This feature comprises sensing notches disposed along the front, right-side and left side edges of the paddle for detecting the relative position of the slots in the stacking rod lying adjacent to the sensing notches.
It is desirable to rapidly transfer semiconductor wafers from holding cassettes to quartz boats before they are introduced into a diffusion furnace, and vice versa, without introducing contamination onto the surfaces of the wafers. Such contamination often results during wafer placement in the quartz boats, from the wafers scraping against the slots into which they are placed. These slots are only slightly wider than the wafers themselves, and thus it has been difficult to achieve sufficiently precise placement as to avoid such scraping altogether.
Misalignment or misorientation of the wafers and the quartz boats occur as each is moved into position during loading or unloading. Wafers will typically move off center while the wafer is removed from the cassette and transferred to the quartz boat. Misaligned wafers will normally scrape against the slots of the quartz boat resulting in particulate contaminants on the surface of the wafers. Scraping can also occur if the quartz boats are misaligned in relation to the paddle such that the wafers contact the slots during loading.
The problem of misalignment of wafers during the loading of quartz boats prior to insertion into a diffusion furnace has been addressed in U.S. Pat. No. 4,682,928 (Foulke et al.), issued July 28, 1987, and U.S. Pat. No. 4,493,606 (Foulke et al.), issued Jan. 15, 1985.
Both of these patents attempt to overcome the problem of misalignment by providing on the paddle a sensor for sensing the alignment of the paddle with some known location, recognizable by a known variation in shape, such as a slot onto which a semiconductor wafer is placed by the paddle. Sensing is done in advance of placement and is accomplished by transmitting an energy beam (e.g., light) along a path intersecting with slots in a holder (paddle) for semiconductor wafers. The output of the sensor is processed to determine each location of the holder at which the wafer paddle is so aligned that a wafer supported thereon would be received at the center of the slots. These alignment locations are stored and used later on to align the holder when wafers are actually transferred to the quartz boat. Two sensors, one for the slots in each of two rails in the holder, are provided on the wafer paddle; and mirrors on a central portion of the paddle reflect light travelling between source and sensors located on outer portions of the paddle, only the central portion actually being received inside the boat.
Foulke et al. '928 and '606 detect the location of the slots by sensing the variation in light intensity at the light sensors. Light travelling from the sources is reflected by mirrors disposed on the paddle back to the sensors disposed on the quartz boat. That light path is blocked except when a slot is encountered. Maximum light transmission occurs at the precise center of each slot. Location of alignment between the paddle and the ten slots are determined from the outputs of the encoder and the light sensors. The ten locations are then processed to determine what wafer rotation angle theta is required to cause wafers carried by the paddle to be aligned with the slots. Once the wafer misalignment angle theta has been determined, the paddle is rotated about the Y axis by precisely that angle.
The following patents also discuss various aspects of wafer misalignment during handling: U.S. Pat. No. 4,765,793 (Goddeau), issued Aug. 23, 1988; U.S. Pat. No. 4,024,944 (Adams et al.), issued May 24, 1977; and U.S. Pat. No. 4,713,551 (Layman et al.), issued Dec. 15, 1987. None of these patents specifically deal with the overcoming of the misalignment of wafers during loading and/or unloading from quartz boats.
Unfortunately, the sensor system disclosed in the Foulke et al. patents does not contemplate the misalignment of the wafer on the paddle itself, nor the combination of quartz boat and wafer misalignment simultaneously. The wafer may be sitting anywhere (i.e., left, right, or in/out) in its cassette due to its handling. Therefore, it is desirable to know where the wafer is when it is picked up by a paddle in order to avoid sliding or bumping as it is loaded into a quartz boat, Moreover, the wafer may also become off center as it is removed from the cassette itself.
The present inventor has developed a unique apparatus and process for overcoming scraping of semiconductor wafers during the loading or unloading of quartz boats due to misalignment of wafers on a paddle, misalignment of the slots in a quartz boat relative to the position of the paddle, or both.